I want to bend the corners of a square sheet to 90 degrees as follows. How should i do it?
The Simple Deform > Bend modifier IMO, is not easy to get your head around.


Create a plane, extending from 0 to its dimension in X and Y, with its origin at world 0.
Cut its outer corner (K, holding Ctrl to constrain to the center of the edges)
Bevel the cut.





Select the inner crease, and (ShiftS), place the 3D cursor there.
Create an empty, rotate to the orientation shown, and put the cursor back where it came from 





Assign a Bend modiifer. The space in which it operates, here, is defined by the empty. From what I've deduced by trial and error, this is what happens, when set as shown:

a) In the modifier's space, rotations of the vertices of the modified object will be around Z.

b) The extent of the modified object in X of the modifier's space is mapped onto a range of 1, with 0 set to the space's origin. So if 2/3 of the object is in negative X, its furthest negative point in X will be mapped to -0.66, and furthest positive to 0.33.

c) Each vertex will be rotated by its mapped position in X, multiplied by the total rotation requested in the modifier.


As far as I can tell, the 'Limits' setting also  refers to a mapped range of 1, but with a different origin. This time, 0 is mapped to the furthest negative point, and 1 to the furthest positive point, in the modifier's X. That's the best way I can explain to myself why the settings shown work the way they do.



Anyway, this is the result:



Now you can add a Mirror (X &amp; Y)  and a Solidify modifier...

 

... parent the empty and the plane to another empty, (here I've raised it in Z, in case you want some 'pendulum' rotations in the animation).. 



For safety, lock off all the transformations of the modifier empty and the plane, placing them under sole control of the parent empty, and you're good to go.